The present invention relates to SOI type (or SOS type) MOS (metal oxide semiconductor) - type transistors with high withstanding voltage.
Offset gate type transistors have been known so far as MOS type transistors provided with high withstanding voltage. Such offset gate type transistor is one to which a lightly doped drain layer having a low impurity concentration and the same conductive type as that of a drain layer has been additionally provided between the drain and the source by ion implantation or the like.
This lightly doped drain layer having a low concentration causes the depletion layer developed while drain voltage is being applied to spread toward the source side, thus diminishing the field strength and contributing to providing with high withstanding voltage. The lightly doped drain layer having a low concentration furthermore serves as a pinch resistance layer, thus allowing the electric current passing across the channel region to flow to the drain electrode by way of this layer.
However, for the reason that the above-mentioned offset gate type transistor forms the lightly doped drain layer having a low impurity concentration separate from a drain layer, one extra mask is required for establishing the lightly doped drain layer having a low impurity concentration, resulting in a drawback that this will mean one additional step to the manufacturing process.